There is a desire to increase data rates for communication of data in wireless communications systems, without adversely affecting other parameters of these systems, such as error rate, complexity, transmitted signal power, and costs.
To this end, it is known to provide a wireless communications system transmitter in which encoded and modulated data to be transmitted is divided into multiple data sub-streams, which are subjected to code spreading using orthogonal codes such as Walsh functions (or code sequences, also referred to as Walsh codes for simplicity), the resulting Walsh coded data sub-streams then being combined and scrambled using a complex PN (pseudo random or pseudo noise) sequence to produce a signal which is amplified in a linear power amplifier (LPA) and transmitted via a single transmit antenna.
For example, in order to provide a data transmission rate of 10.8 Mbps, such a transmitter may use rate 3/4 data encoding, 64 QAM (quadrature amplitude modulation), and 20 data sub-streams and corresponding Walsh codes.
64 QAM provides a relatively high number (6) of bits per modulation symbol, and consequently for a similar signal to noise ratio requires a substantially greater transmit power of the LPA than would be required for lower rate modulation methods such as 16 QAM, 8 PSK (phase shift keying), and QPSK (quadrature PSK) providing respectively 4, 3, and 2 bits per symbol. The LPA constitutes a considerable part of the costs of a transmitter, and this cost increases substantially with increasing transmit signal power, so that it is desirable to use a lower rate modulation method while still providing the desired high data rate.
To this end, it is also known to divide the encoded and modulated data to be transmitted into a larger number of sub-streams, which ale grouped in M groups (M being an integer greater than 1) each of which groups of data sub-streams is processed as described above and supplied via a respective one of M LPAs to a respective one of M transmit antennas. With such a transmitter, for example with M=4 a data transmission rate of 10.8 Mbps can be achieved using rate 1/2 data encoding, QPSK modulation, and 20 data sub-streams and corresponding Walsh codes. The reduced power required of the LPAs due to the lower rate modulation method and the use of 4 LPAs each contributing one quarter of the total transmit signal power can result in a reduction in costs of the transmitter.
At each receiver of such a wireless communications system, each receive antenna receives the signals transmitted from all of the M antennas. In order to separate the data sub-streams at the receiver, it is necessary to process received signals in dependence upon a channel matrix H which must be determined at the receiver. The channel matrix H represents characteristics of each channel or path from each transmit antenna to each receive antenna, and for example can be determined using pilot signals which are also transmitted from the transmitter to the receiver.
The performance of a known system as described above is dependent upon the extent to which the different channels or paths from the M transmit antennas to the receive antennas are uncorrelated, i.e. the extent to which they are subject to fading independently of one another. The less the correlation, the better the performance. However, it has been found that there can be substantial correlation among these paths. In view of this, in such a system it has been necessary for the receiver to have at least M receive antennas, the channel matrix H then being determined as at least an M by M (i.e. M rows and M columns) matrix.
More generally, a known system as described above is required to have at least as many receive antennas as there are transmit antennas. In contrast to this, it would be desirable to provide each receiver with only a relatively small number of receive antennas, for example only two receive antennas, but this would undesirably limit the number of transmit antennas to this small number and would correspondingly limit the advantages of the transmitter described above.
Furthermore, it continues to be desirable to reduce still further the transmit signal power required of the LPAs for a given data rate, or conversely to increase the data rate without similarly increasing the transmit signal power.